1. Field of the Invention
The present invention relates to a photoelectric conversion device manufacturing method, semiconductor device manufacturing method, photoelectric conversion device, and image sensing system.
2. Description of the Related Art
A photoelectric conversion device is recently used in a two-dimensional image input apparatus represented by a digital still camera and a video camcorder or a one-dimensional image reading apparatus represented by a facsimile apparatus and a scanner. The demand for photoelectric conversion devices is rapidly growing.
A photoelectric conversion device uses, for example, a CCD (Charge Coupled Device) or a MOS sensor. CMOS sensors have been in practical use as a typical MOS sensor.
A CMOS sensor includes a pixel array and a control portion.
In the pixel array, a plurality of pixels are arrayed in the row and column directions. Each pixel has a circuit arrangement shown in FIG. 11. FIG. 11 is a circuit diagram of a pixel of a conventional CMOS sensor.
Referring to FIG. 11, a photodiode (to be referred to as a “PD” hereinafter) 1001 converts light into a signal (charges) and accumulates it. A transfer MOS transistor 1002 transfers the signal (charges) accumulated in the PD 1001. Reference numeral 1003 denotes a floating diffusion (to be referred to as an “FD” hereinafter) 1003. A reset signal to reset the FD 1003 and the PD 1001 is supplied to the gate of a reset MOS transistor 1004. A selection signal to select an arbitrary row of the pixel array is supplied from the control portion to the gate of a selection MOS transistor 1005. An amplification MOS transistor 1006 performs a source follower operation together with a constant current source 1008, thereby amplifying a signal received from the FD 1003 and outputting it to a column signal line 1007.
The control portion controls each of the plurality of pixels in the pixel array. The control portion includes at least one of a circuit for processing signals from the pixels, and a driving circuit (shift register) for driving transistors in the pixels, although neither are shown. The control portion is formed as a peripheral circuit in the same substrate as the pixel array.
To implement the circuit arrangement shown in FIG. 11, a technique disclosed in Japanese Patent Laid-Open No. 2006-310650 forms each pixel having a sectional structure shown in FIG. 12. FIG. 12 is a sectional view of a pixel of a conventional CMOS sensor.
As shown in FIG. 12, the PD 1001 includes a charge accumulation layer 1001a and a protection layer 1001b. The charge accumulation layer 1001a is an n-type semiconductor region to accumulate a signal (charges or electrons) generated in accordance with light 1109 that has entered the PD 1001. The protection layer 1001b is a p+-type semiconductor region to protect the surface of the charge accumulation layer 1001a. “P+-type” indicates that the concentration of a p-type impurity is higher than in a “p-type” region.
An element isolation portion 1102 is formed from an insulating film to electrically isolate the charge accumulation layers 1001a of the plurality of PDs 1001 from each other. A channel stop region 1106 that is a p+-type semiconductor region is formed under the element isolation portion 1102. Additionally, a well region 1107 that is a p−-type semiconductor region is formed around the channel stop region 1106 and the charge accumulation layer 1001a. “P−-type” indicates that the concentration of a p-type impurity is lower than in a “p-type” region.
According to Japanese Patent Laid-Open No. 2006-310650, this structure can effectively prevent charge leakage to neighboring pixels.
FIG. 12 illustrates the FD 1003, and a gate 1002a of the transfer MOS transistor 1002.
A recent photoelectric conversion device is required to have more pixels in a predetermined chip area. It is therefore necessary to reduce the area occupied by a unit pixel.
When the size of a pixel having the sectional structure shown in FIG. 12 decreases, the space between the adjacent PDs (photoelectric conversion units) 1001 can also become narrow.
In this case, a signal accumulated in the charge accumulation layer 1001a of the PD 1001 may leak to the charge accumulation layer 1001a of an adjacent PD 1001 via the well region 1107. The well region 1107 is a p−-type semiconductor region and cannot therefore form a sufficient potential barrier against the adjacent charge accumulation layer 1001a. This may lead to a decrease in the sensitivity of the PD 1001.
Additionally, if the charge accumulation layer 1001a of the PD 1001 capacitively couples with the charge accumulation layer 1001a of an adjacent pixel via the well region 1107, the PD 1001 may suffer crosstalk from the charge accumulation layer 1001a of the adjacent pixel.
In a method of manufacturing a general semiconductor device including a photoelectric conversion device, it is difficult to accurately form semiconductor regions having different thicknesses in a predetermined region of a semiconductor substrate.
More specifically, in an ion implantation apparatus for forming a semiconductor region in a predetermined region of a semiconductor substrate, the impurity ion acceleration energy may be restricted by the structure of the ion accelerator. The ion accelerator of an ion implantation apparatus normally accelerates an ionized impurity using a plurality of magnetic field accelerators. Energy the ion implantation apparatus can accelerate in a distance of several meters is only about 1 to 2 MeV, considering the realistic floor area of the apparatus. The ionization efficiency of multi-charged ions decreases exponentially relative to the distance in the traveling direction. For this reason, the number of ions finally reaching near the wafer, that is, the dose further decreases. It is therefore difficult to implant impurity ions to a predetermined depth or more in the semiconductor substrate.
When the ion implantation apparatus increases the acceleration energy, the variation in the impurity concentration in a direction perpendicular to the implantation direction, that is, so-called lateral standard deviation in the semiconductor substrate containing the implanted impurity ions may increase. This may degrade the accuracy in the lateral direction when forming impurity regions having different concentrations in the semiconductor substrate.
Ion implantation into a silicon substrate breaks the single-crystal structure of silicon. For this reason, annealing after implantation for crystal defect recovery and impurity relocation between the lattices are indispensable. In particular, crystal defects in the latter process have a particularly large influence on a photoelectric conversion device. The ion implantation forms, at a high probability, base defects in the semiconductor substrate, for example, a heavy metal that readily forms a deep level. This may cause white defects fatal in an image obtained by the photoelectric conversion device. That is, as the implantation energy rises, the ion implantation amount increases, and the number of times of implantation increases, a higher annealing temperature and a longer process time for defect recovery need be set. This may make it difficult to design a desired profile and result in residual defects.